1. Field of the Invention
This invention relates to digital storage systems in which a flying read/write head is loaded onto the surface of moving storage media with controlled velocity to avoid contact with the surface of the storage media.
2. Description of the Related Art
The use of velocity control to load read/write heads onto the media of Direct Access Storage Devices (DASD) is known. U.S. Pat. No. 4,864,437 describes a head loading velocity control for use in a removable cartridge disk drive. First, a table of voltages at different actuator currents is generated with the actuator locked in a fixed position. Then the velocity is measured by monitoring the total voltage across the actuator, and subtracting the value in the table for the current flowing in the actuator coil. The remainder is treated as the back EMF generated in the coil by actuator motion. A problem with this method of measuring actuator velocity is that the back EMF voltage is more than an order of magnitude smaller than the voltage drops due to coil resistance and other effects. Therefore, any error in the table becomes magnified by the subtraction. Generating the table with the actuator rotor in a fixed position may give a different value of impedance due to circuit bias offsets, current levels and temperature changes.
U.S. Pat. No. 4,480,218 describes a position control system that uses the zero crossings of induced EMF to detect the detent positions of a two-phase stepper motor. In order to use a winding as both drive and position sensor, the current in the winding is reduced to zero before the detent position is reached. This prior art does not sense velocity.
U.S. Pat. No. 4,967,291 describes an over velocity detection system which measures velocity by integrating measured actuator coil current. The measurement is based on the assumption that acceleration is proportional to current and, therefore, the velocity should be proportional to integrated current. A difficulty with this method is that although the acceleration force may be proportional to current, the frictional forces are not constant and, therefore, current may not reflect actuator motion. That is because acceleration force must first overcome friction before the actuator begins to move.
U.S. Pat. No. 4,168,457 describes a method of controlling velocity using feedback signals from the rotating media. This method does not work for measuring head load velocity because the head can not receive feedback signals from the media before it has reached its flying height over the disk media.